Yes, that is the only cost.
The permanent postmarks such as the one @michiel071 has shown are avaliable at any time, they can be issued with any date, including Sundays and Public Holidays.
The pictorial postmarks, usually have timelimit (usually 28 days from the day the bulletin/postmark is avaliable from).
First Day issue Postmarks can only be used on the corresponding stamp, but sponsored Postmarks can be issued on any stamp.
They usually have roughly a 10 day turnaround for postmarking, but a present times I am looking at three to four weeks for postcards/letters to be forwarded or returned.
Edit: I should also add, any item submitted for postmarking should have sufficient postage to cover the return postage, therefore 1st class post is requested, if it is to an international destination, then a postage stamp(s) to that destination should be used, ie £1.70 stamp (or postage stamps equalling that value) for a postcard to Australia, etc